Handle escapes in docstrings more correctly

[penelopeysm]

Closes #1224, but I'm not sure if this is enough? Particularly concerned about escaped quotes

Quotes are handled now

julia> s = raw"""
       \"""
       ```jldoctest
       julia> @macro a.\$s
       [...]
       ```
       \"""
       f
       """
"\"\"\"\n```jldoctest\njulia> @macro a.\\\$s\n[...]\n```\n\"\"\"\nf\n"

julia> ft(s; format_docstrings=true) == s
true

[github-actions]

Docs preview for PR #1228

[penelopeysm]

I don't want to claim that this is perfect because I don't know if I missed something out, but it's definitely an improvement.